I was told by a sales person in the field that using cold-method spray oven cleaner would do the trick, and it works like a charm! I spray it on evenly, let it sit for 15 minutes, then use a scrub brush to clean it. Depending on how bad the floor is, it doesn't take too much effort. Used as a cleaner on a regular basis, it will keep your shower floors (and tubs) looking like new. (Our home is 6 years old, but I have also cleaned other people's houses that were older, and it cleaned up badly stained fiberglass floors there, too.)